Enrichment of anthropogenic organic matter and acetoclastic pathway dominates ebullition CH₄ production in urban river

Methane (CH₄) ebullition emission is recognized as an important contributor to CH₄ fluxes in urban rivers, and understanding ebullition CH₄ production and emission is important to accurately identify the global river carbon budget. However, the current generation pathways of ebullition CH₄ in different types of rivers are still not fully understood. In this study, the isotope technique was used to investigate the production pathways of ebullition CH₄ in different types of rivers through a combination of field investigation and incubation experiment in Shanghai rivers networks to explore the regulating factors of ebullition CH₄ emission. In this study, CH₄ ebullitive fluxes ranged from 18.54 to 2464.24 μmol m⁻² h⁻¹ (mean: 395.72 ± 548.05 μmol m⁻² h⁻¹, median: 151.43 μmol m⁻² h⁻¹), contributing nearly 84 % of total CH₄ emission and varying spatially. High CH₄ ebullitive fluxes were attributed to elevated sediment organic carbon content, which enriched through anthropogenic activities, and shallow water depth. Additionally, CH₄ ebullitive fluxes exhibited temperature sensitivity that was modulated by ecosystem trophic status and organic carbon content. The measured δ¹³C-CH₄ values in bubbles ranged from -68.09 ‰ to -48.23 ‰, corresponding to carbon isotope fractionation factors (α_C) between 1.032 and 1.058, which suggests acetoclastic methanogenesis as the dominant production pathway in all rivers. These findings highlight the escalating role of ebullition CH₄ under climate warming and anthropogenic stress, providing a framework to project future emission dynamics.